WASHINGTON - Disease spread to wild bees from commercially bred bees
used for pollination in agriculture greenhouses may be playing a role
in the mysterious decline in North American bee populations,
researchers said on Tuesday.

Bees pollinate numerous crops, and scientists have been expressing
alarm over their falling numbers in recent years in North America.
Experts warn the bee disappearance eventually could harm agriculture
and the food supply.

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Scientists have been struggling to understand the recent decline in
various bee populations in North America. For example, a virus brought
from Australia has been implicated in massive honeybee deaths last year.

Canadian researchers studied another type of bee, the bumblebee,
near two large greenhouse operations in southern Ontario where
commercially reared pollination bees are used in the growing of crops
such as tomatoes, bell peppers and cucumbers.

The researchers first observed that the commercial bumblebees
regularly flew in and out of vents in the sides of the greenhouses,
escaping from the facilities.

The researchers then devised a mathematical model to predict how
disease might spread from this "spillover" of runaway commercial bees
to their wild cousins.

The model predicted a relatively slow build-up of infection in
nearby wild bumblebee populations over weeks or months culminating in a
burst of transmission generating an epidemic wave that could affect
nearly all of wild bees exposed.

The model also predicted a drop-off in infection rates as you get further from the greenhouses.

GREENHOUSE BUMBLEBEE PARASITES

The researchers then sampled wild bumblebee populations around the
greenhouses, catching bees in butterfly nets, holding them in vials and
taking them back to a laboratory to screen for pathogens, including
testing their feces.

The patterns that had been predicted by their mathematical model were borne out by studying the wild bees, they said.

Most of the parasites in the wild bumblebees were found to be at
normal levels except for one intestinal parasite known as Crithidia
bombi that is common in commercial bee colonies but typically absent in
wild bumblebees.

The researchers found that up to half of wild bumblebees near the greenhouses were infected with this parasite.

"All of the different species of bumblebees that we sampled around
greenhouses showed the same pattern: really high levels of infection
near greenhouses and then declining levels of infection as you moved
out," said Michael Otterstatter of the University of Toronto, one of
the researchers.

"It was quite obvious that this was coming from the greenhouses and
it was a general adverse effect on the bumblebees," Otterstatter added
in a telephone interview.

He said the parasite weakens and often kills bees. The "spillover"
of disease from commercial colonies may be a factor in the decline of
bee populations in North America, he added.